Signal switches are often used to isolate one block of circuitry from another or from the outside world. In this situation, a signal switch can be placed at the end of a transmission line or connected to an external input/output (IO) port. Symptoms of this placement are undershoot and overshoot voltages. Typical signal switches will turn on during undershoot and overshoot events, even when the switch is explicitly turned off, causing errors to data channels connected to the switch's IO ports. Another requirement of signal switches, is when the switch is on, the blocks of circuitry connected to the IO ports need to be connected to each other through low impedance paths. The lower the IO impedance (Ron) the less likely a data error will occur. In addition, Ron variations across the IO voltage range cause distortion to analog signals.
A typical prior art circuit is shown in FIG. 1. The circuit includes inverters INV1, INV2, INV3, and INV4; transistors P0 and N0; input/output nodes A and B; control signal OE; and gate nodes pgate and ngate. Diode connected transistors, parasitic diodes, or explicit diodes Da, Db, and Dc are used to take the voltage on node prail from a diode drop below voltage Vcc to a voltage higher than voltage Vcc if there is an overshoot. Similarly, diodes Dd, De, and Df are used to take the voltage on node nrail from a diode drop above ground Gnd to a voltage lower than ground Gnd if there is an undershoot. While previous solutions protect from overshoot and undershoot, they lack backgate biasing to reduce Ron. A prior art method to reduce the Ron across the IO voltage range is to increase the transmission gate size. However, the increase in capacitance is much greater than the addition of the backgate biasing overshoot undershoot protection circuitry. In addition increasing the transmission gate size does not flatten the Ron versus IO voltage curve, which is important to decreasing signal distortion.